Wireless communication devices are increasingly used in vehicles. In order to allow and/or improve communication of these devices with base stations located outside the vehicle, antennas are mounted on the vehicle. In this connection, efforts are directed to provide only one antenna for all frequency bands used, if possible. The nowadays-used bands of the radio frequency spectrum range from 800 MHz to 5 GHz. Until now, planar inverted-F antennas (PIFA) have been used, which provide a plurality of discrete resonances in their antenna structure. By skillfully designing slots in a flat planar element of the PIFA, it is possible to adjust the radiation pattern of the antenna to the frequency bands needed. However, this requires the PIFA to be adapted as a function of its environment in order to obtain the appropriate radiation pattern because the discrete frequencies shift as a function of the environment. As a result, the antenna design must be adapted for each new vehicle or for a different antenna position on the vehicle.
Another antenna is a circular disk monopole, which includes a circular antenna element disposed in a vertical position with respect to a support. The circular antenna element is connected to the core of a coaxial cable. The radiation pattern of the circular disk monopole has a minimum frequency. Above this minimum frequency, a nearly continuous radiation pattern is obtained for the monopole in the frequency space. Therefore, there is no need for the circular disk monopole to be adapted to the environment. The disadvantage of the circular disk monopole is its vertical design and the space requirements associated therewith. The circular antenna element typically has a diameter of about 10 cm. Also, measures have to be taken to protect the projecting round antenna element against mechanical influences.
An object of the present invention is to provide an improved broadband antenna having a small height.